1. Field of the Invention
The present invention relates generally to wireless sensor networks, and more particularly, to a method for reducing power consumption in a wireless sensor network.
2. Description of the Related Art
In general, a wireless sensor network is installed in an area that is difficult for an operator to manage, such as a comparatively large area, a building, and a structure. The wireless sensor network operates by itself to collect, process, and report environmental information and situation information required by the operator. Therefore, nodes included in the wireless sensor network usually operate using a battery, rather than using an external power supply. However, if there is a node with an exhausted battery in the wireless sensor network, it is often difficult or impossible to replace the exhausted battery of the node with a new battery. Consequently, since the lifetime of the wireless sensor network is determined by the amount of power consumption of a battery, the most important factor to be considered in designing the wireless sensor network is energy efficiency.
Most ad hoc networks including the conventional wireless sensor network employ a Carrier Sense Multiple Access (CSMA) scheme.
The CSMA scheme prevents a transmission error from occurring when two or more nodes simultaneously transmit packets. A corresponding node always monitors whether or not any other nodes are transmitting packets, and transmits a packet only when there are no packets being transmitted by other nodes. If a channel over which a packet is being transmitted is sensed as a result of the monitoring, more specifically, another node is transmitting a packet, then the corresponding node waits for a set time, and then monitors again to determine whether another node is transmitting a packet. Therefore, while a transmitting node using the CSMA scheme can always transmit a packet without restrictions from other conditions when a channel is not used, a receiving node must always be placed in a reception standby state because it does not know when and from which node a transmitted packet will be received.
Among the various operations of a node in a wireless sensor network, the operation of transmitting/receiving a packet is one of the most energy-consuming operations. In a low-speed local area network, such as the wireless sensor network, power required for a node's packet transmission is almost the same as that required for the node's packet reception. Thus, even when the corresponding node is not in the process of packet transmission/reception, but is placed in a reception standby state, it consumes almost the same power as packet transmission/reception. Therefore, in order to reduce the energy consumption of a node in the wireless sensor network, the corresponding node must not be placed in a reception standby state, but in an inactive state for an increased time.
FIG. 1 is a diagram illustrating an operation of transmitting/receiving a packet by each node in a typical wireless sensor network employing the CSMA scheme.
Referring to FIG. 1, portions marked with diagonal lines represent a state where a corresponding node actually participates in packet transmission/reception, shaded portions represent the reception standby state of a corresponding node, and unshaded portions represent the inactive state of a corresponding node.
The portions marked with diagonal lines correspond to portions in which power required for packet transmission/reception is consumed, and all of the shaded portions correspond to portions in which power is wasted. As a result, the power wasted due to the reception standby state increases as the number of nodes increases, or as the density of nodes existing in the same area increases.
In order to reduce the waste of power, the typical wireless sensor network employing the CSMA scheme uses a duty cycling scheme in which active and inactive states are periodically repeated, and communication between nodes is possible only in the active state. In the duty cycling scheme, the ratio of node active state time to the whole time is referred to as “duty cycle”. However, even when the duty cycling scheme is applied to the typical wireless sensor network employing the CSMA scheme, there are limitations in reducing power wasted when each node is placed in a reception standby state.